TECTONIC SUBSIDENCE OF CALIFORNIA ESTUARIES INCREASES FORECASTS OF RELATIVE SEA-LEVEL RISE

Relative sea-level rise is a function of not only changes in the volume of water in the oceans but also the impact of local vertical motion. In this study we calculate subsidence rates in four estuaries (Carpinteria Slough, Goleta Slough, Campus Lagoon, and Morro Bay) along the central California coast by comparing radiocarbon-dated estuarine material to a local regional relative sea-level curve. Subsidence rates range from 1.4+/-1.7 mm/yr, 1.2+/-0.3 mm/yr, and 0.4+/-0.2 mm/yr in Morro Bay, Carpinteria Slough, and Goleta Slough, respectively, to possible uplift in Campus Lagoon (-0.1+/-0.6 mm/yr). In three of four estuaries, high rates of subsidence effectively double the ongoing rates of relative sea-level rise experienced over the last five decades. However within one estuary, the contribution of subsidence to relative sea-level rise is minimal. This difference in subsidence rates is attributed to the geological settings of the estuaries. Estuaries developed in subsiding geological structures such as synclines and fault-bounded basins are subsiding at much higher rates than those developed within incised valleys, drowned river-mouths, and back-barrier lagoons. Restoration projects accounting for future sea-level rise must consider the geologic setting of the estuaries and, if applicable, include subsidence in future sea-level rise scenarios, even along the tectonically uplifting US Pacific Coast.